1. Field of the Invention
The present invention relates to a method of controlling an induction motor for increasing an output torque upon starting operation thereof. Further, the present invention is concerned with an apparatus for carrying out the control method mentioned above.
2. Description of Related Art
According to a hitherto known or conventional scheme for controlling the induction motor, an exciting current component (also referred to simply as the exciting current) and a torque current component (also referred to simply as the torque current) of an AC electric power supplied to the induction motor is controlled independently from each other by resorting to a control method known as the vector control or field-oriented control method. More specifically, upon starting of the induction motor (i.e., when operation of the induction motor is to be started), the exciting current is first supplied to the induction motor. Subsequently, after a sufficient or effective amount of magnetic flux has been generated, the torque current component is supplied to the induction motor for starting the operation of the same.
In addition, there is also known a so-called forcing control method according to which the exciting current of a magnitude which exceeds that of the rated exciting current is forcibly caused to flow through the induction motor for thereby generating the magnetic flux very speedily, i.e., at a very high rate. At the time point when the magnetic flux has reached the rated value, the exciting current is then decreased to the rated value thereof, whereupon the torque current is caused to flow through the induction motor for thereby allowing the operation thereof to be started.
However, in the case of the control apparatus for the induction motor which is equipped with an inverter constituted by a plurality of switching elements connected in the form of a bridge circuit, current limitation is necessarily imposed in view of the current withstanding capability of the switching elements. More specifically, the square root of a sum of a square of the exciting current component and that of the torque current component can not exceed the current limitation value imposed on the switching element. If otherwise, the switching element may unwantedly be injured. Such being the circumstances, there is the necessity of determining proportions of the exciting current component and the torque current component which can ensure a maximum torque within the limited current range. Of course, the attempt for generating the torque of greater magnitude will necessarily be accompanied with the need for employment of expensive switching elements each having a high current capacity and thus being expensive.